Method for producing positive electrode active material for lithium ion secondary batteries, positive electrode active material for lithium ion secondary batteries, and lithium ion secondary battery

ABSTRACT

Making a positive electrode active material for lithium ion secondary batteries includes: weighting and mixing lithium carbonate and a compound containing respective metallic elements other than Li in a composition formula Li α Ni x Co y M1 1−x−y−z M2 z O 2+β  so as to have a metallic constituent ratio of the formula to obtain a mixture, and firing the mixture to obtain a lithium composite compound. Performing, on the mixture, a first heat treatment at 200° C. to 400° C. for 0.5 to 5 hours to obtain a first precursor. A step of performing a heat treatment on the first precursor under an oxidizing atmosphere at 450° C. to 800° C. for 0.5 to 50 hours, and reacting 92 mass % or more of the lithium carbonate to obtain a second precursor, and a finishing step of performing a heat treatment on the second precursor under an oxidizing atmosphere at 755° C. to 900° C. for 0.5 to 50 hours to obtain the lithium composite compound.

TECHNICAL FIELD

The present invention relates to a method for producing a positiveelectrode active material used for a positive electrode of a lithium ionsecondary battery, the positive electrode active material, and a lithiumion secondary battery.

BACKGROUND ART

Conventionally, as typified by, for example, lithium ion secondarybatteries, non-aqueous secondary batteries in which a non-aqueouselectrolyte mediates an electrical conduction between electrodes havebeen used as secondary batteries. The lithium ion secondary battery is asecondary battery in which lithium ions are in charge of electricalconduction between electrodes in charge/discharge reactions. The lithiumion secondary battery features a high energy density and a small memoryeffect compared with other secondary batteries such as a nickel-hydrogenstorage battery and a nickel-cadmium storage battery. Therefore, thelithium ion secondary battery has been expanding its application rangingfrom small power supplies for, for example, mobile electronic devicesand household electrical equipment, stationary power supplies for, forexample, electric power storage devices, uninterruptible power supplysystems, and power leveling devices to medium and large power suppliessuch as driving power supplies for, for example, ships, railways, hybridvehicles, and electric vehicles.

Especially, in the use of the lithium ion secondary batteries as themedium and large power supplies, a high energy density is required forthe batteries. Positive electrodes and negative electrodes are requiredto have a high energy density to achieve the high energy density of thebatteries; therefore, active materials used for the positive electrodeand the negative electrode are required to have a high capacity. As apositive electrode active material having a high charge/dischargecapacity, powder of a lithium composite compound expressed by a chemicalformula of LiM′O₂ (M′ indicates an element such as Ni, Co, and Mn)having an α-NaFeO₂ type layered structure has been known. Sinceexhibiting a trend of increasing the capacity as especially a proportionof Ni increases, this positive electrode active material is expected asthe positive electrode active material achieving the high-energybatteries.

There has been disclosed powder of a lithium-containing compoundexpressed by Li_(a)Ni_(b)M1 _(c)M2 _(d)(O)₂(SO₄)_(X) as such positiveelectrode active material and a method for producing the positiveelectrode active material (see the following Patent Literature 1). Anobject of the invention described in Patent Literature 1 is to providelithium-mixed metal oxide with which secondary particles are not brokenin a battery production (a positive electrode) and do not turn intopowder. As means to achieve the object, the invention configures adifference in D10 values between initial powder and powder aftercompression at 200 MPa measured in accordance with ASTM B 822, thestandard formulated by American Society for Testing and Materials, to be1.0 μm or less.

A production step of the powder of the lithium-containing compounddescribed in Patent Literature 1 includes a step of preparing acoprecipitated nickel-containing precursor having a predeterminedvoidage and a step of mixing the nickel-containing precursor with alithium-containing compound to obtain a precursor mixture. Example ofthis lithium-containing compound includes lithium carbonate, lithiumhydroxide, lithium hydroxide monohydrate, lithium oxide, lithiumnitrate, or a mixture of these substances. The production step furtherincludes a step of heating the obtained precursor mixture to 200 to1000° C. in multiple stages using carrier gas containingCO₂-non-containing (CO₂ content proportion: 0.5 ppm or less) oxygen toproduce a powder product and a step of disintegrating the powder byultrasonic wave and screening the disintegrated powder.

According to Patent Literature 1, a reaction control pertaining to atemperature holding stage in the above-described production step allowsobtaining a product of no aggregation of secondary particles firmlysintered to one another. Patent Literature 1 discloses that this allowsan elimination of a pulverization step that forms square-shapedparticles with squares, which cause particles to break in a materialfloor under a high pressure in an electrode production.

There has been disclosed a production method that allows obtaining ahigh-capacity Li_(y)Ni_((1-x))Mn_(x)O₂ (Here, the numbers of moles of xand y are 0≤x ≤0.3, 1.0≤y≤1.3.) regarding a positive electrode activematerial for non-aqueous electrolyte secondary battery (see thefollowing Patent Literature 2). The production method described inPatent Literature 2 employs a manganese compound equivalent to thenumber of moles of atoms of Mn indicated by x, a nickel compoundequivalent to the number of moles of atoms of Ni indicated by 1-x, and alithium compound equivalent to the number of moles of atoms of Liindicated by y as starting materials. This production method is asynthesis method that performs a first heat treatment afterpreliminarily drying these starting materials, obtains an intermediatethrough a temperature decrease process, and after that performs a secondheat treatment again at a temperature different from that of the firstheat treatment. The production method features that a processingatmosphere at firing is an oxidizing atmosphere (see claim 1 or asimilar description). Patent Literature 2 discloses that the use of theabove-described synthesis method ensures obtaining a positive electrodeactive material for non-aqueous electrolyte secondary battery having ahigh charge/discharge capacity (paragraph 0020).

Patent Literature 2 describes a LiNO₃ hydrate and Li₂CO₃ as examples ofthe lithium compounds as the starting materials (paragraph 0019). Theproduction method described in Patent Literature 2 does not obtain thesynthesis of LiNiO₂ having a space group R-3m structure directly fromthe starting materials, Ni and Li compounds, by a heat treatment, butobtains a final object via the intermediate. Since this intermediate hasan oxygen close packing type similar to a NiO type mainly having arhombohedral structure and further contains Li sites at positions closeto Ni and O atoms, the intermediate is considered to be facilitated tochange to the R-3m structure (see paragraphs 0022 and 0023 and a similardescription). In a determination from X-ray diffraction diagramsillustrated in FIG. 7 and FIG. 9, in the case where, for example, anunreacted Li compound indicative of a crystalline structure differentfrom that of the intermediate is contained, it is determined as improper(see paragraphs 0040 and 0050).

CITATION LIST Patent Literature

Patent Literature 1: JP-T 2010-505732

Patent Literature 2: JP-A H6-96768 A

SUMMARY OF INVENTION Technical Problem

A method for producing a positive electrode active material that fires amixture of lithium carbonate with a compound containing Ni and producesa lithium composite compound with a high Ni concentration has thefollowing problems. The lithium composite compound with the high Niconcentration here means, for example, a lithium composite compoundhaving a layered structure in which an atom ratio (Ni/M′) of Ni to M′ ina chemical formula LiM′O₂ (M′ is a metallic element containing Ni) inexcess of 0.7.

For industrial mass production of the above-described lithium compositeoxide with the high Ni concentration, the synthesis reaction needs to beprogressed by the large amount and uniformly. However, it has been foundthat heating a mixture of lithium carbonate and a compound containing Nigenerates a large amount of carbonic acid gas from the lithiumcarbonate; therefore, the uniform synthesis reaction by the large amountis inhibited. This is because a reverse reaction of the carbonic acidgas with the lithium composite compound with the high Ni concentrationcontaining Ni³⁺ much progresses easily. Additionally, an oxygen partialpressure lowers and a reaction that oxidizes an oxidation number of Nifrom Ni²⁺ to Ni³⁺ is inhibited.

Especially, a problem such as the following has been found out. Thelithium composite compound with the high Ni concentration in which anoxidization of Ni is insufficient substantially lowers the capacity in asecondary battery using a positive electrode containing the lithiumcomposite compound as a positive electrode active material. It has beenfound out that also in the case where, for example, an unreacted Licompound indicative of a crystalline structure different from that ofthe intermediate is not observed in an X-ray diffraction diagram, theremay be a case where a property as the positive electrode active materialcannot be sufficiently obtained, and the oxidation reaction of Ni needsto be progressed with more certainty.

The present invention has been made in consideration of the problems,and an object of the present invention is to provide a method forproducing a positive electrode active material that ensures industriallymass-producing positive electrode active materials containing lithiumcomposite oxide with a high Ni concentration, the positive electrodeactive material, and a lithium ion secondary battery.

Solution to Problem

To achieve the object, a method for producing positive electrode activematerial for lithium ion secondary batteries of the present invention isa method for producing the positive electrode active material used forpositive electrodes of the lithium ion secondary batteries. The methodincludes: a mixing step of weighting and mixing a lithium carbonate anda compound containing respective metallic elements other than Li in thefollowing Formula (1) so as to have a metallic constituent ratio of acomposition formula in the following Formula (1) to obtain a mixture;and a firing step of firing the mixture to obtain a lithium compositecompound expressed by the following Formula (1). The firing stepincludes: a first precursor forming step of performing a heat treatmenton the mixture at a heat treatment temperature of 200° C. or more and400° C. or less for 0.5 hours or more and 5 hours or less to obtain afirst precursor; a second precursor forming step of performing a heattreatment on the first precursor under an oxidizing atmosphere at a heattreatment temperature of 450° C. or more and 800° C. or less for 0.5hours or more and 50 hours or less, the second precursor forming stepreacting 92 mass % or more of the lithium carbonate to obtain a secondprecursor; and a finishing heat treatment step of performing a heattreatment on the second precursor under an oxidizing atmosphere at aheat treatment temperature of 755° C. or more and 900° C. or less for0.5 hours or more and 50 hours or less to obtain the lithium compositecompound.

Li_(α)Ni_(x)Co_(y)M1 _(1−x−y−z)M2_(z)O_(2+β)  (1)

Note that values in the Formula (1) meet: 0.97≤α≤1.08, −0.1≤β≤0.1 ,0.7<x≤0.9, 0.03≤y≤0.3, 0≤z≤0.1, and 0<1−x−y−z, M1 is at least one kindof an element selected from the group consisting of Mn and Al, and M2 isat least one kind of an element selected from the group consisting ofMg, Ti, Zr, Mo, and Nb.

Advantageous Effects of Invention

With the present invention, a positive electrode active materialcontaining lithium composite oxide with a high Ni concentration can beindustrially mass-produced and further a service life of the positiveelectrode active material can be longer than those of conventionalpositive electrode active materials.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart of a method for producing a positive electrodeactive material according to an embodiment of the present invention.

FIG. 2 is a schematic partial cross-sectional view of a secondarybattery containing a positive electrode active material according to theembodiment of the present invention.

FIG. 3 is a graph illustrating compression properties of positiveelectrode active materials according to working examples of the presentinvention.

Description of Embodiments

The following describes embodiments of a method for producing a positiveelectrode active material and the positive electrode active material ofthe present invention in detail with reference to the drawings.

The method for producing a positive electrode active material of thisembodiment is a method to produce the positive electrode active materialused for a positive electrode of a non-aqueous secondary battery such asa lithium ion secondary battery. First, the following describes thepositive electrode active material produced by the method for producingthe positive electrode active material of this embodiment in detail, andnext describes the method for producing the positive electrode activematerial of this embodiment in detail.

(Positive Electrode Active Material)

The positive electrode active material produced by the production methodof this embodiment is a lithium composite compound having an α-NaFeO₂type layered structure and expressed by the following Formula (1). Thepositive electrode active material contains, for example, theabove-described powdery lithium composite compound with a specificsurface area of 0.10 m²/g or more.

Li_(α)Ni_(x)Co_(y)M1_(1−x−y−z)M2_(z)O_(2+β)  (1)

Note that the values in the above-described Formula (1) meet:0.97≤a≤1.08, −0.1≤β≤0.1, 0.7<x<0.9, 0.03≤y≤0.3, 0≤z≤0.1, and 0<1−x−y−z,M1 is at least one kind of an element selected from the group consistingof Mn and Al, and M2 is at least one kind of an element selected fromthe group consisting of Mg, Ti, Zr, Mo, and Nb.

The positive electrode active material made of the lithium compositecompound having the α-NaFeO₂ type layered structure expressed by theabove-described Formula (1) allows repetitive reversible insertion anddetachment of lithium ions in association with charge and discharge andhas a low resistance.

Particles of the lithium composite compound constituting the positiveelectrode active material may be primary particles where individualparticles are separated, may be secondary particles where the pluralityof primary particles are bonded by sintering or a similar method, or maybe primary particles or secondary particles containing a free lithiumcompound.

The primary particles of the positive electrode active materialpreferably have an average grain diameter of, for example, 0.1 μm ormore and 2 μm or less. Designing the average grain diameter of theprimary particles of the positive electrode active material to be 2 μmor less improves a filling property of the positive electrode activematerial in the positive electrode when the positive electrodecontaining the positive electrode active material is produced, therebyensuring producing the positive electrodes with a high energy density.From a similar aspect, the secondary particles of the positive electrodeactive material preferably have the average grain diameter of, forexample, 3 μm or more and 50 μm or less.

By granulating the primary particles or the secondary particles of thepositive electrode active material produced by the method for producingthe positive electrode active material described later by drygranulation or wet granulation, the average grain diameter of thesecondary particles can be adjusted. For example, a granulator such as aspray dryer and a tumbling fluidized bed device is available asgranulation means.

In the above-described Formula (1), αindicates a content ratio of Li.The higher the content ratio of Li is, the higher a valence oftransition metal before charge is, and the rate of change in the valenceof the transition metal at Li detachment is lowered, ensuring improvingcharge/discharge cycle characteristics of the positive electrode activematerial. On the contrary, the higher the content ratio of Li is, thelower the charge/discharge capacity of the positive electrode activematerial is. Accordingly, the range of α indicative of an amount ofexcess/deficiency of Li in the above-described Formula (1) is designedto be 0.97 or more and 1.08 or less. This ensures improving thecharge/discharge cycle characteristics of the positive electrode activematerial and reducing the decrease in the charge/discharge capacity.

More preferably, the range of a indicative of the content ratio of Li inthe above-described Formula (1) can be designed to be 0.98 or more and1.05 or less. As long as α in the above-described Formula (1) is 0.98 ormore, the amount of Li sufficient to contribute to the charge anddischarge is secured and the high-capacity positive electrode activematerial can be achieved. Additionally, as long as a in theabove-described Formula (1) is 1.05 or less, a charge compensation canbe sufficiently secured in case of the change in valence of thetransition metal, and both the high capacity and the highcharge/discharge cycle characteristics can be satisfied.

With x indicative of the content ratio of Ni in the above-describedFormula (1) larger than 0.7, the amount of Ni preferable to contributeto the charge and discharge can be secured in the positive electrodeactive material, thereby ensuring achieving the high capacity.Meanwhile, with x in the above-described Formula (1) in excess of 0.9, apart of Ni is replaced by a Li site. This fails to secure the amount ofLi sufficient to contribute to the charge and discharge and possiblylowers the charge/discharge capacity of the positive electrode activematerial. Accordingly, by designing x indicative of the content ratio ofNi in the above-described Formula (1) in the range of larger than 0.7 to0.9 or less and more preferably in a range of larger than 0.75 to 0.85or less, the positive electrode active material can have the highcapacity and also the decrease in charge/discharge capacity can bereduced.

Additionally, as long as y indicative of the content ratio of Co in theabove-described Formula (1) is 0.03 or more, this ensures contributingto stabilization of the layered structure of the positive electrodeactive material. Stably maintaining the layered structure allowsreducing a cation mixing that mixes, for example, Ni into the Li sites;therefore, the excellent charge/discharge cycle characteristics can beobtained. Meanwhile, y in the above-described Formula (1) in excess of0.3 relatively increases the ratio of Co, which is limited in the supplyamount and has the high cost, being disadvantageous in terms ofindustrial production of the positive electrode active materials.Accordingly, by designing y indicative of the content ratio of Co in theabove-described Formula (1) in the range of 0.03 or more to 0.3 or less,and more preferably in the range of larger than 0.05 to 0.2 or less, thecharge/discharge cycle characteristics of the positive electrode activematerial can be improved, being advantageous in terms of the industrialmass production of the positive electrode active materials.

M1 in the above-described Formula (1) is at least one kind or more ofelements selected from the group consisting of Mn and Al. Adding Mn orAl, or Mn and Al together provides an effect of stably maintaining thelayered structure even when Li is detached by charging. However, 1-x-y-zindicative of a content ratio of at least one kind or more of elementsselected from the group consisting of Mn and Al in the above-describedFormula (1) of 0.30 or more lowers the capacity of the positiveelectrode active material.

M1 in the above-described Formula (1) is preferably Mn. This is because,with the M1 in the above-described Formula (1) being Mn, the layeredstructure can be further stably maintained even when Li is detached bycharging and the capacity higher than the case of M1 being Al can beobtained. Designing 1-x-y-z indicative of the content ratio of Mn whenM1 is Mn to be 0.04 or more ensures decreasing an average valence of Niin LiM′O₂. Therefore, even when the oxidation reaction of Ni does notsufficiently progress, the reaction indicated in the following Formula(2) progresses, a reaction temperature lowers, and a reaction of lithiumcarbonate in a second precursor forming step described later ispromoted. Since Al can take tetravalent similarly to Mn, an effectsimilar to that of Mn can be expected.

Li₂CO₃+2M′O+0.5O₂→2LiM′O₂+CO₂  (2)

Note that M′ in the above-described Formula (2) indicates an elementsuch as Ni, Co, and Mn. As described above, the promotion of thereaction of the lithium carbonate in the second precursor forming steplowers lithium carbonate melted and becoming a liquid phase in afinishing heat treatment step described later. This lowers an amount ofliquid phase in the finishing heat treatment step and reduces a growthof crystal grains, making a high-temperature firing possible.

Furthermore, with M1 being Mn, designing 1-x-y-z indicative of thecontent ratio of Mn to be 0.04 or more allows the charge/dischargereactions even when a crystallite diameter and a primary particlediameter of the positive electrode active material are large, making thehigh heat treatment temperature in the finishing heat treatment steppossible. Consequently, the Ni oxidation reaction is promoted in thefinishing heat treatment step, a lithium compound remaining on a surfacecan be reduced, and lithium ions in the layered structure arestabilized.

With M1 being Mn, 1-x-y-z indicative of the content ratio of Mn inexcess of 0.18 lowers the capacity of the positive electrode activematerial. Additionally, with M1 being Mn, designing 1-x-y-z indicativeof the content ratio of Mn to be 0.10 or more allows the heat treatmenttemperature in the finishing temperature treatment step to be furtherhigh, and therefore is preferable. Accordingly, with M1 being Mn,1-x-y-z indicative of the content ratio of Mn preferably meets 0.04≤1-x-y-z≤0.18.

Additionally, with M1 being Mn, y/(1-x-y-z) indicative of the ratio ofthe content ratios of Co to Mn is preferably 0.1 or more and 3 or less.y/(1-x-y-z) of 0.1 or more maintains the content ratio of Co in thepreferable range and contributes to the stabilization of the layeredstructure. As long as y/(1-x-y-z) is 3 or less, the high reactiontemperature of the above-described Formula (2) can be reduced and thereaction of the lithium carbonate can be sufficiently progressed in thesecond precursor forming step described later.

With M2 in the above-described Formula (1) containing at least one kindof a metallic element selected from the group consisting of Mg, Ti, Zr,Mo, and Nb, an electrochemical activity of the positive electrode activematerial can be enhanced. Replacing metal sites of the positiveelectrode active material with these metallic elements ensures improvingstability of the crystalline structure of the positive electrode activematerial and an electrochemical property (such as the charge/dischargecycle characteristics) of the layered positive electrode activematerial. With z indicative of the content ratio of M2 in theabove-described Formula (1) in excess of 0.1 lowers the capacity of thepositive electrode active material. Accordingly, designing the range ofz in the above-described Formula (1) to be 0 or more and 0.1 or lessensures further reducing the decrease in capacity of the positiveelectrode active material.

βin the above-described Formula (1) indicates an allowable range of alayered structure compound belonging to a space group R-3m and excess ordeficiency amount of oxygen. The range of β in the above-describedFormula (1) being the range of −0.1 or more and 0.1 or less ensuresmaintaining the layered structure of the positive electrode activematerial.

Additionally, a weight of lithium carbonate remaining on the surface ofthe positive electrode active material after the finishing heattreatment step is preferably 0.2 mass % or less. Designing the weight ofthe lithium carbonate remaining on the surface to be 0.2 mass % or lessensures reducing an amount of carbonic acid gas generated by lithiumcarbonate degradation caused by the charge/discharge cycles and ensuresimproving the charge/discharge cycle characteristics. The weight of thelithium carbonate remaining on the surface of the positive electrodeactive material after the finishing heat treatment step can be adjustedby, for example, performing a water washing step.

The weight of the lithium hydroxide remaining on the surface of thepositive electrode active material is preferably 0.7 mass % or less. Thelithium hydroxide generates hydrofluoric acid (HF) that exhibits strongacid through reaction to fluorine-based electrolyte contained in theelectrolyte of the secondary battery. Further, the lithium hydroxidepromotes an oxidative decomposition of the electrolyte by a highvoltage. This deteriorates the performance of the secondary battery andmakes it difficult to obtain the satisfactory charge/discharge cyclecharacteristics. Therefore, by designing the lithium hydroxide remainingon the surface of the positive electrode active material to be 0.7 mass% or less ensures obtaining the secondary battery having thesatisfactory charge/discharge cycle characteristics.

The lithium compound remaining on the surface of the positive electrodeactive material can be quantitated by, for example, a Titration Method,a Temperature Programmed Desorption-Mass Spectrometry (TPD-MS), and IonChromatography (IC). The crystalline structure of the particles of thepositive electrode active material can be confirmed by, for example, anX-ray diffraction method (XRD). An average composition of the particlesof the positive electrode active material can be confirmed by, forexample, an Inductively Coupled Plasma (ICP) and an Atomic AbsorptionSpectrometry (AAS).

The positive electrode active material of this embodiment is thepositive electrode active material used for the positive electrode ofthe lithium ion secondary battery and features the following. Thespecific surface area is formed of 0.10 m²/g or more of the lithiumcomposite compound expressed by the above-described Formula (1). Anamount of dissolution of the lithium hydroxide is 0.33 mass % or less.In other words, the positive electrode active material of thisembodiment is the positive electrode active material used for thepositive electrode of the lithium ion secondary battery and features thefollowing. The specific surface area is formed of 0.10 m²/g or more ofthe lithium composite compound expressed by the above-described Formula(1). An dissolution speed of the lithium hydroxide is 0.22 mass %/h orless. Here, the amount of dissolution of the lithium hydroxide is adifference between an amount of lithium hydroxide A and an amount oflithium hydroxide B, which is (B−A) mass %. The amount of lithiumhydroxide A is detected after immersion of the positive electrode activematerial into pure water for 30 minutes at a solid content percentage of1.6 mass % by neutralization titration. The amount of lithium hydroxideB is detected after immersion of the positive electrode active materialinto pure water for 120 minutes at a solid content percentage of 1.6mass % by neutralization titration. The dissolution speed correspondingto this amount of dissolution is (B−A) mass %/1.5 h. The amount ofdissolution of the lithium hydroxide is preferably less than 0.30 mass %and the dissolution speed of the lithium hydroxide is preferably lessthan 0.2 mass %/h.

By designing the specific surface area of the positive electrode activematerial to be 0.10 m²/g or more, the average grain diameters of theprimary particles and the secondary particles of the positive electrodeactive material can fall within the above-described preferable ranges.The specific surface area of the positive electrode active material ispreferable to be 2.0 m²/g or less. This ensures improving the fillingproperty of the positive electrode active material in the positiveelectrode and producing the positive electrode with the high energydensity.

The positive electrode active material having the specific surface areaof 0.8 m²/g or more and 1.2 m²/g or less is more preferable. Thespecific surface area of the positive electrode active material is aspecific surface area that can be measured using, for example, anautomatic specific surface area measuring apparatus and calculated by aBET method.

As long as the amount of dissolution (B−A) of the lithium hydroxide inthe positive electrode active material of 0.33 mass % or less or thedissolution speed of the lithium hydroxide is 0.22 mass %/h or less, thelayered structure is stabilized, thus ensuring obtaining thesatisfactory charge/discharge cycle characteristics. Meanwhile, with theamount of dissolution (B−A) of the lithium hydroxide in the positiveelectrode active material in excess of 0.33 mass % or the dissolutionspeed of the lithium hydroxide in excess of 0.22 mass %/h, the layeredstructure is destabilized, making it difficult to obtain thesatisfactory charge/discharge cycle characteristics.

A particle fracture strength of the positive electrode active materialis preferably 10 MPa or more and 200 MPa or less. This does not fracturethe particles of the positive electrode active material in the processof manufacturing the electrodes and reduces a poor coating such aspeeling when slurry containing the positive electrode active material iscoated over the surface of a positive electrode current collector toform a positive electrode mixture layer. The particle fracture strengthper particle of the positive electrode active material can be measuredusing, for example, a microcompression testing machine.

A molding density of the positive electrode active material at a presspressure of 5 MPa is preferably 2.5 g/cm³ or more. How much the electricenergy can be accumulated per unit volume depends on how the electrodedensity is configured to be a high density. When the molding densities,which are densities when powder molding is performed at the identicalpressure, are compared, as the molding density becomes high, the powderis better in compressibility. The compressibility of the positiveelectrode active material can be evaluated by, for example, performing acompression test with an autograph and measuring the molding density.

(Method for Producing Positive Electrode Active Material)

Next, the following describes the method for producing the positiveelectrode active material of this embodiment that produces theabove-described positive electrode active material. FIG. 1 is aflowchart illustrating respective steps included in the method forproducing the positive electrode active material of this embodiment.

The method for producing the positive electrode active material of thisembodiment is a method for producing the positive electrode activematerial used for the positive electrode in the lithium ion secondarybattery. The method for producing the positive electrode active materialof this embodiment mainly includes a mixing step S1 and a firing stepS2. The mixing step S1 weights and mixes the lithium carbonate and thecompound containing the respective metallic elements other than Li inthe above-described Formula (1) so as to be the metallic constituentratio of the composition formula of the above-described Formula (1) toobtain a mixture. The firing step S2 fires the mixture obtained at themixing step Si to obtain the lithium composite compound expressed byFormula (1).

The mixing step S1 can employ a compound containing the metallicelements other than Li in the above-described Formula (1), for example,a Ni-containing compound, a Co-containing compound, an Mn-containingcompound, an Al-containing compound, and an M2-containing compound asstarting materials of the positive electrode active material, inaddition to the lithium carbonate. The M2-containing compound is acompound containing at least one kind of a metallic element selectedfrom the group consisting of Mg, Ti, Zr, Mo, and Nb.

At the mixing step S1, the starting material weighted at the proportionso as to be the predetermined element composition corresponding to theabove-described Formula (1) is mixed to prepare raw material powder. Themethod for producing the positive electrode active material of thisembodiment uses the lithium carbonate as the starting materialcontaining Li. Compared with other Li-containing compounds, such aslithium acetate, lithium nitrate, lithium hydroxide, lithium chloride,and lithium sulfate, the lithium carbonate is excellent in stability insupply, a low cost, and weak alkali; therefore, damage to manufacturingequipment is little and industrial utilization and usefulness areexcellent.

As the Ni-containing compound, the Co-containing compound, theMn-containing compound, and the Al-containing compound as the startingmaterials of the positive electrode active material, for example, oxide,hydroxide, carbonate, hydrosulfate, or acetate is available, and the useof oxide, hydroxide, or carbonate is especially preferable. As theM2-containing compound, for example, acetate, nitrate, carbonate,hydrosulfate, oxide, or hydroxide is available, and the use ofcarbonate, oxide, or hydroxide is especially preferable.

At the mixing step S1, the starting material is preferably pulverizedby, for example, a pulverizer and mixed. This ensures preparing auniformly-mixed powdery solid mixture. The starting material ispreferably pulverized to have the average grain diameter of 0.3 μm orless, and further preferably pulverized to have the average graindiameter of 0.15 μm or less. As the pulverizer, which pulverizes thecompound of the starting material, a general precision pulverizer suchas a ball mill, a jet mill, and a sand mill is available. A granulationstep can employ, for example, a spray drying method. Various kinds ofmethods such as a two-fluid or a four-fluid nozzle and a disk type areemployable as a spray method in the spray drying method.

The firing step S2 is a step of obtaining the lithium composite compoundexpressed by the above-described Formula (1) through firing of themixture obtained at the mixing step Si and includes a first precursorforming step S21, a second precursor forming step S22, and a finishingheat treatment step S23.

The first precursor forming step S21 obtains a first precursor through aheat treatment of the mixture at a heat treatment temperature of 200° C.or more and 400° C. or less for 0.5 hours or more and 5 hours or less.The main purpose of the first precursor forming step S21 is to remove avaporized component such as water content, which inhibits a synthesisreaction of the positive electrode active material, from the mixtureobtained at the mixing step S1. That is, the first precursor formingstep S21 is a heat treatment step for dehydration that removes the watercontent in the mixture.

At the first precursor forming step S21, the vaporized componentcontained in the heat-treated mixture such as water content, impurities,and a volatile component in association with a pyrolysis, for example,evaporates, burns, and volatilizes, and gas is generated. Since theheat-treated mixture contains carbonate such as the lithium carbonate,carbonic acid gas in association with a pyrolysis of the carbonate isalso generated in the first precursor forming step S21.

At the first precursor forming step S21, the heat treatment temperatureof less than 200° C. possibly results in insufficient burning reactionof the impurities and insufficient pyrolysis reaction of the startingmaterial. At the first precursor forming step S21, the heat treatmenttemperature in excess of 400° C. possibly forms the layered structure ofthe lithium composite compound under an atmosphere containing the gasgenerated from the mixture by the heat treatment. Accordingly, the heattreatment of the mixture at the heat treatment temperature of 200° C. ormore and 400° C. or less in the first precursor forming step S21 ensuressufficiently removing the vaporized component such as the water contentand obtaining the first precursor in which the layered structure has notbeen formed yet.

As long as the heat treatment temperature is in the range of 250° C. ormore and 400° C. or less and more preferably 250° C. or more and 380° C.or less in the first precursor forming step S21, the removal effect ofthe vaporized component such as the water content and the effect ofreducing the formation of the layered structure can be further improved.A heat treatment period in the first precursor forming step S21 can beappropriately changed according to, for example, the heat treatmenttemperature, a degree of removal of the vaporized component, and adegree of reduction of the formation of the layered structure.

The first precursor forming step S21 preferably performs the heattreatment in an air current of atmosphere gas and under exhaust with apump in order to exhaust the gas generated from the heat-treatedmixture. A flow rate of the atmosphere gas per minute or an amount ofexhaust per minute with the pump is preferably larger than the volume ofthe gas generated from the mixture. The volume of the gas generated fromthe heat-treated mixture in the first precursor forming step S21 can becalculated based on, for example, a ratio of the vaporized component tothe mass of the starting material contained in the mixture.

The first precursor forming step S21 may be performed under a reducedpressure at an atmospheric pressure or less. Since the main purpose ofthe first precursor forming step S21 is not the oxidation reaction, anoxidizing atmosphere in the first precursor forming step S21 may be theatmosphere. The use of the atmosphere as the oxidizing atmosphere in thefirst precursor forming step S21 allows simplifying a configuration of aheat treatment apparatus, facilitating the supply of the atmosphere,improving the productivity of the positive electrode active materials,and lowering the production cost. The atmosphere for the heat treatmentin the first precursor forming step S21 is not limited to the oxidizingatmosphere and may be a non-oxidizing atmosphere such as inert gas.

While the firing step S2 performs the second precursor forming step S22after the termination of the first precursor forming step S21, theoxidizing atmosphere used at the first precursor forming step S21 may beexhausted after the termination of the first precursor forming step S21,and the second precursor forming step may be performed by introducing anew oxidizing atmosphere. Thus performing the gas replacement allowspreventing the gas generated from the mixture of the starting materialby the heat treatment at the first precursor forming step S21 fromaffecting the second precursor forming step S22. The first precursor maybe once taken out from the heat treatment apparatus after the firstprecursor forming step S21, and the first precursor may be put into theheat treatment apparatus again. In the case where the exhaust isperformed at the heat treatment or after the heat treatment in the firstprecursor forming step S21, the first precursor forming step S21 and thesecond precursor forming step S22 may be consecutively performed.

The second precursor forming step S22 performs the heat treatment on thefirst precursor obtained at the first precursor forming step S21 at theheat treatment temperature of 450° C. or more and 800° C. or less for0.5 hours or more and 50 hours or less under the oxidizing atmosphereand causes 92 mass % or more of the lithium carbonate to react, thusobtaining a second precursor. The main purpose of the second precursorforming step S22 is to transform the lithium carbonate in the firstprecursor into lithium oxide, cause the lithium carbonate to react totransition metal, synthesize a compound having a layered structureexpressed by a composition formula LiM′O₂, and remove a carbonic acidcomponent. That is, the second precursor forming step S22 is a heattreatment step that removes the carbonic acid component in the firstprecursor.

To develop the positive electrode active material with the high Niconcentration in which the range of x indicative of the content ratio ofNi in the above-described Formula (1) is larger than 0.7 and 0.9 or lessinto the high capacity, the valence of Ni needs to be oxidized from abivalence to a trivalent in the firing step S2. The bivalent Ni easilyoccupies the Li positions in the layered structure LiM′O₂, causing adecrease in the capacity of the positive electrode active material.Therefore, the firing step S2 fires the mixture obtained at the mixingstep Si under the oxidizing atmosphere to change an oxidation number ofNi from Ni²⁺ to Ni³⁺. The carbonic acid gas inhibits the progress of thereaction of the above-described Formula (2), casing the low capacity ofthe positive electrode active material. Therefore, the firing step S2preferably performs the firing under the atmosphere not containing thecarbonic acid gas as much as possible.

To promote the Ni oxidation reaction at the finishing heat treatmentstep S23, the second precursor forming step S22 degrades the lithiumcarbonate as the main carbonic acid gas source to lower an amount ofgenerated carbonic acid gas in the finishing heat treatment step S23 asmuch as possible. To promote the reaction of the above-described Formula(2), the atmosphere for the heat treatment in the second precursorforming step S22 is an oxidizing atmosphere containing oxygen and theoxygen concentration is preferably 80% or more, the oxygen concentrationof 90% or more is more preferable, the oxygen concentration of 95% ormore is further preferable, and the oxygen concentration of 100% is yetfurther preferable. The carbonic acid gas concentration under theatmosphere for the heat treatment in the second precursor forming stepS22 is preferably 5% or less, 1% or less is more preferable, and 0.1% orless is further preferable. For successive progress of the reaction ofthe above-described Formula (2), consecutively supplying the oxygen atthe heat treatment in the second precursor forming step S22 ispreferable, and performing the heat treatment in the air current of theoxidizing atmosphere gas is preferable.

To smoothly progress the Ni oxidation reaction in the second precursorin the finishing heat treatment step S23, the second precursor formingstep S22 needs to sufficiently lower a residue derived from the startingmaterial. Accordingly, the second precursor forming step S22 reacts 92mass % or more of the lithium carbonate contained in the mixture weighedand mixed so as to have the metallic constituent ratio of thecomposition formula of the above-described Formula (1). When the secondprecursor forming step S22 reacts 92 mass % or more of the lithiumcarbonate contained in the mixture, the amount of generated carbonicacid gas in the finishing heat treatment step S23 can be lowered, andthe reaction of the above-described Formula (2) and the oxidationreaction of Ni can be promoted.

Furthermore, when the second precursor forming step S22 reacts 92 mass %or more of the lithium carbonate contained in the mixture, an amount ofliquid phase of the lithium carbonate being melted and becoming theliquid phase is lowered in the finishing heat treatment step S23, and agrowth of crystal grains is reduced, making the high temperature firingpossible. Performing the finishing heat treatment step S23 at a highertemperature promotes the Ni oxidation reaction; therefore, the lithiumcompound remaining on the surface can be reduced, and the lithium ionsin the layered structure are stabilized. Consequently, the positiveelectrode active material having the satisfactory charge/discharge cyclecharacteristics is obtained. The second precursor forming step S22preferably reacts 97 mass % or more of the lithium carbonate containedin the mixture. When the second precursor forming step S22 reacts 97mass % or more of the lithium carbonate contained in the mixture, theamount of generated carbonic acid gas can be further lowered in thefinishing heat treatment step S23 and the positive electrode activematerial having the more satisfactory charge/discharge cyclecharacteristics can be obtained.

In the case where lithium salt other than the lithium carbonate is usedas a part of the starting material of the lithium contained in thepositive electrode active material, a proportion of the lithium presentas the lithium carbonate is preferably less than 7 mole % among thelithium components in the second precursor. This allows the amount ofgenerated carbonic acid gas to be lowered in the finishing heattreatment step S23, and the reaction of the above-described Formula (2)and the oxidation reaction of Ni can be promoted. Additionally, in thiscase, the amount of liquid phase of the lithium carbonate is lowered andthe growth of the crystal grains is reduced, making the high temperaturefiring possible in the finishing heat treatment step S23. As describedabove, the positive electrode active material having the satisfactorycharge/discharge cycle characteristics can be obtained.

In the case where lithium salt other than the lithium carbonate is usedas a part of the starting material of the lithium contained in thepositive electrode active material, a proportion of the lithium presentas the lithium carbonate is more preferably less than 3 mole % among thelithium components in the second precursor. Accordingly, the amount ofgenerated carbonic acid gas can be further lowered in the finishing heattreatment step S23 and the positive electrode active material having themore satisfactory charge/discharge cycle characteristics can beobtained.

The heat treatment temperature in the second precursor forming step S22of less than 450° C. results in considerably slow progress of theformation reaction of the layered structure and excessively remaininglithium carbonate while the first precursor is heat-treated to form thesecond precursor having the layered structure. Meanwhile, the heattreatment temperature in the second precursor forming step S22 in excessof 800° C. excessively progresses the grain growth, failing to obtainthe high-capacity positive electrode active material. Accordingly, theheat treatment temperature in the second precursor forming step S22 ispreferably higher than 550° C., more preferably 600° C. or more and 700°C. or less, and further preferably a high temperature, 650° C. or moreand 680° C. or less. Since the reaction of 92 mass % or more of thelithium carbonate and preferably 97 mass % or more of the lithiumcarbonate can be further promoted, the heat treatment temperature and/orthe ratio of Mn in the second precursor forming step S22 is preferablyset to high. Specifically, setting the heat treatment temperature in thesecond precursor forming step S22 to be higher than 550° C. ensures thefurther promoted reaction of the lithium carbonate. In the case where M1in the above-described Formula (1) is Mn and 1-x-y-z is larger than 0and smaller than 0.075, the heat treatment temperature is preferably600° C. or more, and in the case where 1-x-y-z is 0.075 or more, theheat treatment temperature is preferably higher than 550° C. The highratio of Mn allows decreasing the average valence of Ni in LiM′O₂. Evenwhen the oxidation reaction of Ni does not sufficiently progress, thereaction indicated by the above-described Formula (2) progresses and thereaction temperature lowers; therefore, the reaction of the lithiumcarbonate in the second precursor forming step S22 is promoted.Therefore, in the case where M1 in the above-described Formula (1) is Mnand 1-x-y-z is larger than 0 and smaller than 0.075, the heat treatmenttemperature is set to 600° C. or more, and in the case where 1-x-y-z is0.075 or more, the heat treatment temperature is set to be higher than550° C. This ensures the reaction of 92 mass % or more of the lithiumcarbonate contained in the mixture and therefore is preferable.Meanwhile, setting the heat treatment temperature in the secondprecursor forming step S22 to be 700° C. or less ensures reducinggeneration of the liquid phase and further improving the reductioneffect of the growth of the crystal grains.

To fully react the first precursor to the oxygen in the temperaturerange of the heat treatment in the second precursor forming step S22,the period of the heat treatment can be set to 0.5 hours or more and 50hours or less. From an aspect of promoting the reaction of the lithiumcarbonate, the period of the heat treatment in the second precursorforming step S22 is preferably two hours or more and 50 hours or less.From an aspect of improving the productivity, the period of the heattreatment in the second precursor forming step S22 is more preferablytwo hours or more and 15 hours or less.

While the firing step S2 performs the finishing heat treatment step S23after the termination of the second precursor forming step S22, theoxidizing atmosphere used at the second precursor forming step S22 maybe exhausted after the termination of the second precursor forming stepS22, and the finishing heat treatment step S23 may be performed byintroducing a new oxidizing atmosphere.

This allows preventing the gas generated by the heat treatment in thesecond precursor forming step S22 from affecting the finishing heattreatment step S23. The second precursor may be once taken out from theheat treatment apparatus after the termination of the second precursorforming step S22, and the second precursor may be put into the heattreatment apparatus again. In the case where the exhaust is performed atthe heat treatment or after the heat treatment in the second precursorforming step S22, the second precursor forming step S22 and thefinishing heat treatment step S23 may be consecutively performed. Thesecond precursor forming step can use, for example, a continuousconveyance furnace and a rotary kiln.

The finishing heat treatment step S23 performs the heat treatment on thesecond precursor obtained in the second precursor forming step S22 atthe heat treatment temperature of 755° C. or more and 900° C. or lessfor 0.5 hours or more and 50 hours or less under the oxidizingatmosphere, thus obtaining the lithium composite compound. The lithiumcomposite compound obtained in this finishing heat treatment step S23constitutes the positive electrode active material of this embodiment.The main purpose of the finishing heat treatment step S23 is to grow thecrystal grains to fully progress the Ni oxidation reaction, whichoxidizes Ni in the second precursor obtained at the second precursorforming step S22 from the bivalence to the trivalent, and to developelectrode performance of the lithium composite compound obtained by theheat treatment. That is, the finishing heat treatment step S23 is a heattreatment step that performs the Ni oxidation reaction in the secondprecursor and the crystal grain growth.

To fully progress the Ni oxidation reaction in the second precursor inthe finishing heat treatment step S23, the atmosphere for the heattreatment in the finishing heat treatment step S23 is an oxidizingatmosphere containing oxygen.

In the oxidizing atmosphere in the finishing heat treatment step 23, theoxygen concentration is preferably 80% or more, the oxygen concentrationof 90% or more is more preferable, the oxygen concentration of 95% ormore is further preferable, and the oxygen concentration of 100% is yetfurther preferable. The carbonic acid gas concentration under theatmosphere for the heat treatment in the second precursor forming stepS22is preferably 5% or less, 1% or less is more preferable, and 0.1% orless is further preferable.

The heat treatment temperature in the finishing heat treatment step S23of less than 755° C. possibly makes the progress of the crystallizationof the second precursor difficult. The heat treatment temperature inexcess of 900° C. fails to reduce the degradation of the layeredstructure in the second precursor and generates the bivalent Ni, therebylowering the capacity of the obtained lithium composite compound.Accordingly, setting the heat treatment temperature in the finishingheat treatment step S23 to 755° C. or more and 900° C. or less promotesthe grain growth of the second precursor. Additionally, the degradationof the layered structure is reduced and the capacity of the obtainedlithium composite compound can be improved. When the heat treatmenttemperature in the finishing heat treatment step S23 is set to be higherthan 800° C., preferably 840° C. or more and 890° C. or less, and morepreferably in excess of 850° C. and 890° C. or less, the promotioneffect of the grain growth and the degradation reduction effect of thelayered structure can be further improved.

A low oxygen partial pressure in the finishing heat treatment step S23promotes the Ni oxidation reaction and therefore heat is required.Accordingly, in the case where the oxygen supply to the second precursoris insufficient in the finishing heat treatment step S23, the heattreatment temperature needs to be increased. The increase in the heattreatment temperature cannot avoid the degradation of the layeredstructure in the obtained lithium composite compound, failing to obtainthe satisfactory electrode property of the positive electrode activematerial. Accordingly, to fully supply the oxygen to the secondprecursor in the finishing heat treatment step S23, the period of theheat treatment in the finishing heat treatment step S23 can be set to0.5 hours or more and 50 hours or less. From an aspect of improving theproductivity of the positive electrode active material, the period ofthe heat treatment in the finishing heat treatment step S23 ispreferably 0.5 hours or more and 15 hours or less.

From an aspect of the production process, the obtained lithium compositecompound is preferably not washed with water after the finishing heattreatment step S23. While performing the water washing allows decreasingthe lithium compound remaining on the surface, the water possiblyextracts the lithium in the positive electrode active material otherthan the lithium remaining on the positive electrode active materialsurface, leading to deterioration of the property of the positiveelectrode active material. The water content remained after the waterwashing degenerates a binder during the positive electrode productionand possibly causes the poor coating. Entering the remaining watercontent in the battery causes the water content to react to theelectrolyte. This generates hydrogen fluoride and possibly deterioratesthe battery property.

However, it has been found that, from an aspect of performanceimprovement in the positive electrode active material, the water washingof the obtained lithium composite compound after the finishing heattreatment step S23 is advantageous. That is, it has been found out thatproviding the step of water washing after the finishing heat treatmentstep S23 improves the compression property of the powder of the positiveelectrode active material. This will be described later.

As described above, in the method for producing the positive electrodeactive material of this embodiment, the firing step S2, which fires themixture obtained at the mixing step S21, includes the first precursorforming step S21, the second precursor forming step S22, and thefinishing heat treatment step S23. This ensures obtaining the firstprecursor in which the vaporized component, mainly such as the watercontent, has been removed from the mixture in the first precursorforming step S21. Then, the second precursor forming step S22 performsthe heat treatment on the first precursor to fully generate the carbonicacid gas, and reacts 92 mass% or more of the lithium carbonate in thesecond precursor, thereby ensuring obtaining the second precursor inwhich the generation of the carbonic acid gas by heating is reduced.

The reaction of 92 mass % or more of the lithium carbonate in the secondprecursor allows the high heat treatment temperature in the finishingheat treatment step S23. Consequently, the oxidation reaction of Ni ispromoted, the oxidation number of Ni changes from Ni²⁺ to Ni³⁺, thelithium compound remaining on the surface of the positive electrodeactive material can be lowered, and the lithium ions in the layeredstructure are stabilized. Therefore, the positive electrode activematerial having the satisfactory charge/discharge cycle characteristicscan be obtained.

Furthermore, the high heat treatment temperature becomes possible in thefinishing heat treatment step S23, and the generation of the carbonicacid gas from the second precursor is reduced. Accordingly, the lowoxygen partial pressure under the oxidizing atmosphere is suppressed,the large amount of Ni oxidation reaction in the second precursoruniformly progresses, and the growth of the crystal grains progresses.Accordingly, the method for producing the positive electrode activematerial of this embodiment can decrease the bivalent Ni remaining inthe lithium composite compound with the high Ni concentration having thelayered structure, change the bivalent Ni to the trivalent Ni, andobtain the high-capacity positive electrode active material excellent ina capacity retention rate.

The method for producing the positive electrode active material of thisembodiment brings the significant effect when the weight of the producedpositive electrode active material becomes a large amount, for example,several hundred g or more. This is because, when the weight of theproduced positive electrode active material is several g, the influencefrom the gas generated from the starting material in the firing step S2is small; however, when the positive electrode active materials aremass-produced in an industrial scale, the volume of the gas generatedfrom the starting material in the firing step S2 increases, and this islikely to lower the oxygen partial pressure under the oxidizingatmosphere.

When the first precursor forming step S21 is omitted in the firing stepS2, the oxygen partial pressure lowers in the second precursor formingstep S22 and the finishing heat treatment step S23. As a result, inorder to sufficiently progress the formation reaction of the layeredstructure of the lithium composite compound, which involves theoxidation of Ni, the finishing heat treatment would need to be performedat a higher temperature, and therefore the preferable temperature rangewould be exceeded. Additionally, when the second precursor forming stepS22 is omitted, the grain growth in the lithium composite compoundprogresses with the insufficient oxidation reaction of Ni and thereforethe omission is not preferable. Further, omitting the finishing heattreatment step S23 fails to obtain the appropriate electrode property.

(Positive Electrode and Lithium Ion Secondary Battery)

The following describes a positive electrode for a non-aqueous secondarybattery using the positive electrode active material produced by theabove-described method for producing the positive electrode activematerial and a configuration of the non-aqueous secondary battery thatincludes the positive electrode. FIG. 2 is a schematic partialcross-sectional view of a positive electrode 111 of this embodiment anda non-aqueous secondary battery 100 including the positive electrode111.

The non-aqueous secondary battery 100 of this embodiment is, forexample, a cylindrical lithium ion secondary battery. The non-aqueoussecondary battery 100 includes a cylindrical battery can 101 with aclosed bottom that houses non-aqueous electrolyte, a wound electrodegroup 110 housed in the battery can 101, and a circular plate-shapedbattery lid 102 that seals an upper opening of the battery can 101. Thebattery can 101 and the battery lid 102 are, for example, manufacturedof a metallic material such as stainless steel and aluminum. The batterylid 102 is fixed to the battery can 101 via a sealing material 106 madeof a resin material having an insulating property by a crimping or asimilar method. This seals the battery can 101 with the battery lid 102,and the battery can 101 and the battery lid 102 are mutuallyelectrically insulated. The shape of the non-aqueous secondary battery100 is not limited to the cylindrical shape, and any other shapes suchas a square shape, a button shape, and a laminated sheet shape areemployable.

The wound electrode group 110 is manufactured by winding a longstrip-shaped positive electrode 111 and negative electrode 112, whichare opposed via a long strip-shaped separator 113, around the windingcenter axis. In the wound electrode group 110, a positive electrodecurrent collector 111a is electrically connected to the battery lid 102via a positive electrode lead piece 103, and a negative electrodecurrent collector 112a is electrically connected to a bottom portion ofthe battery can 101 via a negative electrode lead piece 104. Aninsulating plate 105 that prevents short-circuit is located between thewound electrode group 110 and the battery lid 102 and between the woundelectrode group 110 and the bottom portion of the battery can 101. Thepositive electrode lead piece 103 and the negative electrode lead piece104 are members for current extraction manufactured of materials similarto those of the positive electrode current collector 111a and thenegative electrode current collector 112 a, respectively, and are joinedto the positive electrode current collector 111a and the negativeelectrode current collector 112 a by a spot welding, an ultrasonicpressure welding, or a similar method, respectively.

The positive electrode 111 of this embodiment includes the positiveelectrode current collector 111 a and a positive electrode mixture layer111 b, which is formed on the surface of the positive electrode currentcollector 111 a. As the positive electrode current collector 111 a, forexample, a metal foil, an expanded metal, and a perforated metal madeof, for example, aluminum or an aluminum alloy are available. The metalfoil can be configured to have a thickness of, for example, around 15 μmor more and 25 μm or less. The positive electrode mixture layer 111 bcontains the positive electrode active material produced by theabove-described method for producing the positive electrode activematerial. The positive electrode mixture layer 111 b may contain aconductive material, a binder, or a similar material.

The negative electrode 112 includes the negative electrode currentcollector 112a and a negative electrode mixture layer 112 b, which isformed on the surface of the negative electrode current collector 112 a.As the negative electrode current collector 112 a, for example, a metalfoil, an expanded metal, and a perforated metal made of, for example,copper or a copper alloy or nickel or a nickel alloy are available. Themetal foil can be configured to have a thickness of, for example, around7 μm or more and 10 μm or less. The negative electrode mixture layer 112b contains a negative electrode active material used for the generallithium ion secondary batteries. The negative electrode mixture layer112 b may contain a conductive material, a binder, or a similarmaterial.

As the negative electrode active material, for example, one kind or moreof materials such as a carbon material, a metallic material, and a metaloxide material are available. As the carbon material, for example,graphites such as natural graphite and artificial graphite, carbidessuch as coke and pitch, amorphous carbon, and carbon fiber areavailable. As the metallic material, lithium, silicon, tin, aluminum,indium, gallium, magnesium, and an alloy of these substances, and as themetal oxide material, metal oxide containing, for example, tin, silicon,lithium, and titanium are available.

As the separator 113, for example, a polyolefin-based resin such aspolyethylene, polypropylene, and polyethylene-polypropylene copolymer, amicroporous film such as polyamide resin and aramid resin, and nonwovenfabric are available.

The positive electrode 111 and the negative electrode 112 can beproduced through, for example, a mixture preparing step, a mixturecoating step, and a molding step. The mixture preparing step uses, forexample, stirring means such as a planetary mixer, a dispersion mixer,and a rotating and revolving mixer to stir and homogenize a positiveelectrode active material or a negative electrode active materialtogether with solution containing, for example, a conductive materialand a binder to prepare mixture slurry.

As the conductive material, a conductive material used for the generallithium ion secondary batteries is available. Specifically, for example,carbon particles such as graphite powder, acetylene black, furnaceblack, thermal black, and channel black, and carbon fiber are availableas the conductive material. The conductive material by the amount of,for example, around 3 mass % or more and 10 mass % or less with respectto the entire mass of the mixture is available.

As the binder, a binder used for the general lithium ion secondarybatteries is available. Specifically, for example, polyvinylidenefluoride (PVDF), polytetrafluoroethylene, poly hexafluoropropylene,styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile,and modified polyacrylonitrile are available as the binder. The binderby the amount of, for example, around 2 mass % or more and 10 mass % orless with respect to the entire mass of the mixture is available. Themixing ratio of the negative electrode active material to the binder isdesirable to be, for example, 95:5 by the weight ratio.

A solvent of the solution is selectable from, for example, N-methylpyrrolidone, water, N,N-dimethylformamide, N,N-dimethylacetamide,methanol, ethanol, propanol, isopropanol, ethylene glycol, diethyleneglycol, glycerin, dimethylsulfoxide, and tetrahydrofuran according tothe kind of the binder.

The mixture coating step first applies the mixture slurry containing thepositive electrode active material and the mixture slurry containing thenegative electrode active material prepared at the mixture preparingstep over the surfaces of the positive electrode current collector 111 aand the negative electrode current collector 112 a, respectively, bycoating means such as a bar coater, a doctor blade, and a roll transfermachine. Next, the respective positive electrode current collector 111 aand negative electrode current collector 112 a over which the mixtureslurries have been applied are heat-treated to volatile or vaporize thesolvents in the solutions contained in the mixture slurries to removethe solvents. Thus, the positive electrode mixture layer 111 b and thenegative electrode mixture layer 112 b are formed on the surfaces of thepositive electrode current collector 111 a and the negative electrodecurrent collector 112 a, respectively.

The molding step first performs compression molding on the respectivepositive electrode mixture layer 111 b, which is formed on the surfaceof the positive electrode current collector 111 a, and negativeelectrode mixture layer 112 b, which is formed on the surface of thenegative electrode current collector 112 a, for example, usingpressurizing means such as a roll press. This ensures configuring thepositive electrode mixture layer 111 b so as to have a thickness around,for example, 100 μm or more and 300 μm or less, and the negativeelectrode mixture layer 112 b so as to have a thickness around, forexample, 20 μm or more and 150 μm or less. Afterwards, the positiveelectrode current collector 111 a and the positive electrode mixturelayer 111 b, and the negative electrode current collector 112 a and thenegative electrode mixture layer 112 b are each cut out into the longstrip shape, thus ensuring producing the positive electrode 111 and thenegative electrode 112.

Here, as described above, in the case where the step of the waterwashing of the obtained lithium composite compound is provided after thefinishing heat treatment step S23 in the production step of the positiveelectrode active material, compressibility of the positive electrodemixture layer 111 b, which is formed on the surface of the positiveelectrode current collector 111 a, can be improved. More specifically,water-washing and drying the lithium composite compound allows modifyingthe surface of the lithium composite compound and improving thecompressibility of the positive electrode active material. This allowsimproving the density of the positive electrode mixture layer 111 b andincreasing electric energy accumulated per unit volume.

The following describes a water washing/drying step that performs thewater washing and the drying of the lithium composite compound in moredetails.

The water washing/drying step immerses the lithium composite compoundinto pure water, removes the liquid by a solid-liquid separation, anddries the remaining solid material. These substances may be stirred whenthe lithium composite compound is immersed into the pure water. Addingthe pure water such that the solid content percentage falls within therange of 33 mass % to 77 mass % is preferable for such water washing.The solid content percentage higher than 77 mass % makes the uniformwater washing difficult. The solid content percentage lower than 33 mass% results in an excessive decreased amount of the lithium in thepositive electrode active material, possibly deteriorating the propertyof the positive electrode active material. Since the effect appears inan extremely short period, the immersion period during which the lithiumcomposite compound is immersed into the pure water is preferably within20 minutes and more preferably within ten minutes. The immersion periodin excess of 20 minutes decreases the lithium in the positive electrodeactive material, possibly deteriorating the property of the positiveelectrode active material.

As the solid-liquid separation, various kinds of methods such as afiltration under reduced pressure, a filtration under pressure, a filterpress, a roller press, and a centrifuge are available. A moisturepercentage of the lithium composite compound after the solid-liquidseparation is preferably 20 mass % or less and 10 mass % or less is morepreferable. The moisture percentage of the lithium composite compound inexcess of 20 mass % increases an amount of reprecipitation of thelithium compound dissolved into the water content on the positiveelectrode active material and increases the water content remainingafter the drying. This loses coatability when the positive electrodeactive material is applied over the positive electrode current collectorand worsens the battery property, and therefore is not preferable. Themoisture percentage of the lithium composite compound after thesolid-liquid separation can be measured with, for example, an infraredmoisture meter.

In the drying step of the lithium composite compound after the waterwashing, the atmosphere in the drying step is selectable from, forexample, in the air in which partial pressures of the water vapor andcarbon dioxide are lowered, in nitrogen, in oxygen, or in vacuum. Thedrying step is especially preferable to be performed in the vacuum. Adrying temperature in the drying step is preferably 150° C. or more and300° C. or less and more preferably 190° C. or more and 250° C. or less.The drying temperature in the drying step of lower than 150° C. makes itdifficult to fully remove the water content, and the drying temperaturehigher than 300° C. makes a side reaction that worsens the property ofthe positive electrode active material remarkable.

The drying step preferably dividedly dries the lithium compositecompound after the water washing twice or more. For example, before thetemperature of the lithium composite compound after the water washing isincreased to 150° C. or more, removing the most water content at atemperature around 60° C. to 100° C. is preferable. By removing the mostwater content at the low temperature, the side reaction in thesubsequent drying at a high temperature is lowered, ensuring reducing anegative effect to the property of the lithium composite compound. Themoisture percentage of the lithium composite compound after the dryingis preferably 400 ppm or less, more preferably 300 ppm or less, andfurther preferably 250 ppm or less. The moisture percentage of thelithium composite compound after the drying can be measured by a KarlFischer's method.

The positive electrode 111 and the negative electrode 112 produced asdescribed above are opposed via the separator 113 and wound around thewinding center axis to be the wound electrode group 110. In the woundelectrode group 110, the negative electrode current collector 112 a iscoupled to the bottom portion of the battery can 101 via the negativeelectrode lead piece 104, the positive electrode current collector 111 ais coupled to the battery lid 102 via the positive electrode lead piece103, the insulating plate 105 or a similar member prevents the batterycan 101 and the battery lid 102 from short-circuiting, and the woundelectrode group 110 is housed in the battery can 101. Afterwards, thenon-aqueous electrolyte is injected to the battery can 101, the batterylid 102 is fixed to the battery can 101 via the sealing material 106,and the battery can 101 is sealed, thus ensuring producing thenon-aqueous secondary battery 100.

As the electrolyte injected to the battery can 101, the use ofelectrolyte produced by dissolving lithium hexafluorophosphate (LiPF₆),lithium tetrafluoroborate (LiBF₄), lithium perchlorate (LiCLO₄), or asimilar substance as the electrolyte into a solvent such as diethylcarbonate (DEC), dimethyl carbonate (DMC), ethylene carbonate (EC),propylene carbonate (PC), vinylene carbonate (VC), methyl acetate (MA),ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC) isdesirable. The electrolyte desirably has the concentration of 0.7 M ormore and 1.5 M or less. A compound having a carboxylic acid anhydridegroup, a compound containing elemental sulfur such as propanesultone,and a compound containing boron may be mixed with these electrolytes.The objects to add these compounds are to reduce reductive decompositionof the electrolyte on the surface of the negative electrode, preventreduction precipitation at the negative electrode of the metallicelement such as manganese eluted from the positive electrode, improve anion conductive property of the electrolyte, provide incombustibility ofthe electrolyte, and a similar object; therefore, the compound onlyneeds to be appropriately selected according to the object.

The non-aqueous secondary battery 100 having the above-describedconfiguration includes the battery lid 102 as a positive electrodeexternal terminal and the bottom portion of the battery can 101 as anegative electrode external terminal. The non-aqueous secondary battery100 can accumulate electric power supplied from the outside at the woundelectrode group 110 and the electric power accumulated to the woundelectrode group 110 can be supplied to an external device or a similardevice. Thus, the non-aqueous secondary battery 100 of this embodimentis, for example, available as a small power supply for, for example, amobile electronic device and household electrical equipment, astationary power supply for, for example, an uninterruptible powersupply and a power leveling device, and a driving power supply for, forexample, a ship, a railway, a hybrid vehicle, and an electric vehicle.

The following describes Working Examples based on the method forproducing the positive electrode active material, the positive electrodeactive material, and the lithium ion secondary battery of the presentinvention and Comparative Examples different from the method forproducing the positive electrode active material, the positive electrodeactive material, and the lithium ion secondary battery of the presentinvention.

Working Example 1 (Mixing Step)

First, lithium carbonate, nickel hydroxide, cobalt carbonate, andmanganese carbonate were prepared as the starting materials of thepositive electrode active material. Next, the respective startingmaterials were weighted so as to meet: Li:Ni:Co:Mn=1.04:0.80:0.10:0.10by an atom ratio, pulverized by a pulverizer, and a wet blending wasperformed to prepare slurry.

(Firing step: First Precursor Forming Step, Second Precursor FormingStep)

Next, the slurry (mixture) obtained at the mixing step was dried by aspray dryer, and the dried mixture was fired to obtain fired powder.Specifically, the mixture of 300 g produced by drying the slurryobtained at the mixing step was filled in an alumina container with 300mm in length, 300 mm in width, and 100 mm in height. A heat treatmentwas performed on the mixture under an air atmosphere at a heat treatmenttemperature of 350° C. for one hour in a continuous conveyance furnace(a first precursor forming step). Next, a heat treatment was performedon the powder (the first precursor) obtained in the first precursorforming step in an oxygen air current at the heat treatment temperatureof 575° C. for ten hours in the continuous conveyance furnace whoseatmosphere was replaced by an atmosphere with an oxygen concentration inthe furnace of 99% or more (a second precursor forming step).

(Measurement of Amount of Reacted Lithium Carbonate in Second Precursor)

The amount of reacted lithium carbonate in the powder (the secondprecursor) obtained in the second precursor forming step was analyzed bya neutralization titration as follows. First, the second precursor of0.2 g was dispersed into pure water of 30 ml bubbled with argon gas, andthe pure water was stirred for 60 minutes. Afterwards, the pure waterinto which the second precursor had been dispersed was suctioned andfiltered to obtain filtrate. The obtained filtrate was titrated withhydrochloric acid.

The titration curve goes through two stages, a curve up to a firstequivalence point indicates a total amount of hydroxide ion in lithiumhydroxide and carbonate ion in lithium carbonate, and a curve from thefirst equivalence point to a second equivalence point indicates anamount of carbonic acid hydrogen ion generated from the carbonate ion.Therefore, the amount of lithium carbonate was calculated from a titerfrom the first equivalence point to the second equivalence point. Theamount of the lithium hydroxide was calculated from a difference betweenthe titer up to the first equivalence point and the titer from the firstequivalence point to the second equivalence point. Based on thecalculated amount of lithium carbonate, the amount of reacted lithiumcarbonate in the second precursor was obtained by the following Formula(3).

{(Q ₀ −Q ₂)/Q ₂}×100=Q _(R)  (3)

In the above-described Formula (3), Q₀ indicates the amount of lithiumcarbonate in the starting material, Q₂ indicates the amount of lithiumcarbonate in the second precursor, and Q_(R) indicates the amount ofreacted lithium carbonate in the second precursor. Here, the amount oflithium carbonate Q₀ in the starting material can be calculated, forexample, from the mixing ratio of the starting material.

(Firing Step: Finishing Heat Treatment Step)

Next, the heat treatment was performed on the second precursor obtainedin the second precursor forming step in oxidation air current at theheat treatment temperature of 865° C. for ten hours in the continuousconveyance furnace whose atmosphere was replaced by the atmosphere withan oxygen concentration in the furnace of 99% or more to obtain firedpowder (a lithium composite compound). The obtained fired powder wasclassified by openings of 53 μm or less to produce positive electrodeactive materials. This Working Example did not perform the water washingon the obtained lithium composite compound after the finishing heattreatment step.

(Measurements of Amount of Remaining Lithium Hydroxide and Amount ofRemaining Lithium Carbonate in Positive Electrode Active Material)

The amount of remaining lithium hydroxide and the amount of remaininglithium carbonate on the positive electrode active material of WorkingExample 1 obtained by the above-described steps were analyzed by theneutralization titration as follows. First, similarly to the measurementof the amount of reacted lithium carbonate in the second precursor, thepositive electrode active material of 0.5 g was dispersed into purewater of 30 ml bubbled with argon gas, and the pure water was stirredfor 60 minutes. Afterwards, the pure water into which the secondprecursor had been dispersed and stirred was suctioned and filtered toobtain filtrate. The obtained filtrate was titrated with hydrochloricacid. Similarly to the amount of lithium hydroxide and the amount oflithium carbonate in the second precursor, the amount of remaininglithium hydroxide and the amount of remaining lithium carbonate on thepositive electrode active material were calculated by a titration of thefiltrate produced by immersing the positive electrode active materialinto the pure water and stirring the pure water for 60 minutes.

(Measurement of Amount of Dissolution of Lithium Hydroxide in PositiveElectrode Active Material)

The amount of dissolution of the lithium hydroxide in the positiveelectrode active material was measured by the following procedure.First, the positive electrode active material with the solid contentpercentage of 1.6 mass % was immersed and stirred into pure water for 30minutes, and then an amount of lithium hydroxide A was detected by aneutralization titration of the filtrate. The identical positiveelectrode active material with the solid content percentage of 1.6 mass% was immersed and stirred into pure water for 120 minutes, and then anamount of lithium hydroxide B was detected by the neutralizationtitration of the filtrate. Then, an amount of dissolution (B−A), whichis a difference between the amount of lithium hydroxide A and the amountof lithium hydroxide B, was obtained. The dissolution speed can beobtained by dividing the amount of dissolution by the immersion period.

(Measurements of Lattice Constant and Crystallite Diameter of PositiveElectrode Active Material)

The crystallite diameter of the positive electrode active material wasmeasured by the following procedure. First, the crystalline structure ofthe positive electrode active material was measured by X-ray diffraction(XRD) to obtain the lattice constant of the positive electrode activematerial. The XRD measurement was performed by a concentration methodusing an XRD measurement device manufactured by Rigaku Corporation,RINT-2000. The CuKa line was used for the X-ray, and the output was setto 48 kV and 28 mA.

With the measuring conditions, a step width was set to 0.02°, themeasurement period per step was set to one second, and the measurementresult was smoothened by a Savitzky-Golay method. Afterwards, thebackground and the Kα₂ line were removed to obtain a (003) peak at thetime and a half-value width β_(exp) p of (104). Furthermore, ahalf-value width β_(i) when a standard Si sample (NIST Standard Material640d) was measured in the identical device and under the identicalconditions was obtained, and the half-value width β was defined by thefollowing Formula (4).

[Expression 1]

β=√{square root over (β_(exp) ²−β_(l) ²)}  (4)

Using this half-value width β, the crystallite diameter was obtainedusing the Scherrer formula expressed by the following Formula (5).

$\begin{matrix}\left\lbrack {{Expression}\mspace{14mu} 2} \right\rbrack & \; \\{D = \frac{K\; \lambda}{\beta \; \cos \; \theta}} & (5)\end{matrix}$

In the above-described Formula (5), λ indicates the wavelength of theX-ray, θ indicates the reflection angle, and K indicates the Scherrerconstant, and K=0.9 was met. Then, an average value of the crystallitediameters at the (003) peak and the (104) peak was set as thecrystallite diameter of the positive electrode active material ofWorking Example 1.

(Composition of Positive Electrode Active Material and Measurement ofSpecific Surface Area)

Furthermore, the composition of the positive electrode active materialwas analyzed by ICP using an ICP emission spectrophotometer manufacturedby PerkinElmer Inc., OPTIMA8300. As the pretreatment, the positiveelectrode active material was dissolved into aqua regia, andquantitative analysis was performed on the dissolved solution by theICP. Furthermore, the measurement was performed using an automaticspecific surface area measuring apparatus BELCAT manufactured by BELJAPAN, INC., and the specific surface area of the positive electrodeactive material was calculated by the BET method.

(Evaluation for Compressibility of Positive Electrode Active Material)

The compressibility of the positive electrode active material wasevaluated by measurement with the autograph AGS-X manufactured byShimadzu Corporation. Positive electrode active material powder of 0.5 gwas filled in a mold with φ10 mm, and the mold was installed to theautograph. The positive electrode active material was measured at acrosshead speed of a speed of 1.0 mm/min and up to a test force of 4000N to obtain a compression curve of the powder from the test force at thetime, the amount of stroke, the cross-sectional area of the mold, andthe weight of the positive electrode active material. Thecompressibility was evaluated from the molding density at the presspressure of 5 MPa in this compression curve.

Working Example 2

Except that the heat treatment temperature in the second precursorforming step was set to 600° C. and the heat treatment temperature inthe finishing heat treatment step was set to 842° C., a positiveelectrode active material of Working Example 2 was produced similarly tothe positive electrode active material of

Working Example 1, and the measurements were performed similar to thoseof the positive electrode active material of Working Example 1.

From Working Example 3 to Working Example 6

Except that the heat treatment temperature in the finishing heattreatment step was set to 850° C. for Working Example 3, 865° C. forWorking Example 4, 880° C. for Working Example 5, and 895° C. forWorking Example 6 in the production steps of the positive electrodeactive materials, the positive electrode active materials of WorkingExample 3 to Working Example 6 were produced similarly to the positiveelectrode active material of Working Example 2, and the measurementswere performed similar to those of the positive electrode activematerial of Working Example 1. Additionally, when measured using amicrocompression testing machine (MCT-510 manufactured by ShimadzuCorporation), the particle fracture strength was 66 MPa in WorkingExample 3, 61 MPa in Working Example 4, 58 MPa in Working Example 5, and57 MPa in Working Example 6. The particle fracture strength with themicrocompression testing machine was measured by sparging the positiveelectrode active material on a pressure plate by a minute amount andcompressing the positive electrode active material in units of oneparticle at the test force of 49 mN and the load rate of 0.4747 mN/sec.

Working Example 7

In the mixing step, the respective starting materials were weighted soas to meet Li:Ni:Co:Mn =1.04:0.80:0.125:0.075 by the atom ratio. In thefiring step, the heat treatment temperature in the second precursorforming step was set to 600° C. and the heat treatment temperature inthe finishing heat treatment step was set to 850° C. Except that, apositive electrode active material of Working Example 7 was producedsimilarly to the positive electrode active material of Working

Example 1 and the measurements were performed similar to those of thepositive electrode active material of Working Example 1.

Working Example 8

In the mixing step, the respective starting materials were weighted soas to meet Li:Ni:Co:Mn =1.08:0.80:0.05:0.15 by the atom ratio. In thefiring step, the heat treatment temperature in the second precursorforming step was set to 600° C. and the heat treatment temperature inthe finishing heat treatment step was set to 865° C. Except that, apositive electrode active material of Working Example 8 was producedsimilarly to the positive electrode active material of Working Example 1and the measurements were performed similar to those of the positiveelectrode active material of Working Example 1.

Working Example 9

In the mixing step, the respective starting materials were weighted soas to meet Li:Ni:Co:Mn =1.04:0.80:0.15:0.05 by the atom ratio. In thefiring step, the heat treatment temperature in the finishing heattreatment step was set to 755° C. Except that, a positive electrodeactive material of Working Example 9 was produced similarly to thepositive electrode active material of Working Example 2 and themeasurements were performed similar to those of the positive electrodeactive material of Working Example 1.

Working Example 10

Except that the heat treatment temperature in the finishing heattreatment step was set to 770° C., a positive electrode active materialof Working Example 10 was produced similarly to the positive electrodeactive material of Working Example 9 and the measurements were performedsimilar to those of the positive electrode active material of WorkingExample 1.

From Working Example 11 to Working Example 13

In the mixing step, the respective starting materials were weighted soas to meet Li:Ni:Co:Mn =1.08:0.80:0.15:0.05 by the atom ratio. In thefiring step, the heat treatment temperature in the second precursorforming step was set to 690° C. Except that the heat treatmenttemperature in the finishing heat treatment step was set to 820° C. forWorking Example 11, 850° C. for Working Example 12, and 880° C. forWorking Example 13, positive electrode active materials of WorkingExample 11 to Working Example 13 were produced similarly to the positiveelectrode active material of Working Example 1, and the measurementswere performed similar to those of the positive electrode activematerial of Working Example 1. Additionally, when measured using themicrocompression testing machine (MCT-510 manufactured by ShimadzuCorporation), the particle fracture strength was 71 MPa in WorkingExample 9, 132 MPa in Working Example 10, and 125 MPa in Working Example11.

Working Example 14 and Working Example 15

Working Example 14 and Working Example 15 were obtained by performingthe water washing and the drying processes on the positive electrodeactive materials obtained in Working Example 13 and Working Example 5,respectively. The water washing and the drying processes were performedby the following procedure. The positive electrode active materials werewater-washed by being immersed in pure water with the solid contentpercentage of 43 mass % and stirred at room temperature for 20 minutes.The positive electrode active materials were filtered and then weredried in vacuum at 190° C. for ten hours.

Working Example 16

A positive electrode active material of Working Example 16 was obtainedby performing the water washing and the drying processes on the positiveelectrode active material obtained in Working Example 13. The waterwashing and the drying processes of the positive electrode activematerial were performed by the following procedure. First, the positiveelectrode active material obtained in Working Example 13 was immersedinto pure water with the solid content percentage of 66 mass % at roomtemperature for ten seconds and was water-washed. Next, the positiveelectrode active material that had been immersed into the pure water andon which the water washing had been completed was depressurized andfiltered. Afterwards, the drying was performed in vacuum in two stages:drying at the drying temperature of 80° C. for 14 hours and furtherdrying at the drying temperature of 190° C. for 14 hours.

Working Example 17

Except that the drying temperature at the second stage was set to 240°C. in Working Example 16, a positive electrode active material ofWorking Example 17 was manufactured similarly to the positive electrodeactive material of Working Example 16.

Working Example 18

Except that the drying at the first stage in Working Example 17 was notperformed and only the drying at the second stage was performed, apositive electrode active material of Working Example 18 wasmanufactured similarly to the positive electrode active material ofWorking Example 17.

Comparative Example 1

In the mixing step, the respective starting materials were weighted soas to meet Li:Ni:Co:Mn=1.04:0.80:0.15:0.05 by the atom ratio. In thefiring step, the heat treatment temperature in the second precursorforming step was set to 550° C. and the heat treatment temperature inthe finishing heat treatment step was set to 755° C. Except that, apositive electrode active material of Comparative Example 1 was producedsimilarly to the positive electrode active material of Working Example 9and the measurements were performed similar to those of the positiveelectrode active material of Working Example 1.

Comparative Example 2

Except that the heat treatment temperature in the second precursorforming step was set to 575° C. and the heat treatment temperature inthe finishing heat treatment step was set to 755° C., a positiveelectrode active material of Comparative Example 2 was producedsimilarly to the positive electrode active material of ComparativeExample 1, and the measurements were performed similar to those of thepositive electrode active material of Working Example 1.

Comparative Example 3

Except that the heat treatment temperature in the finishing heattreatment step was set to 906° C., a positive electrode active materialof Comparative Example 3 was produced similarly to the positiveelectrode active material of Working Example 2 and the measurements wereperformed similar to those of the positive electrode active material ofWorking Example 1.

The following Table 1 shows the composition formulae of the positiveelectrode active materials, the heat treatment temperatures in thesecond precursor forming step, the heat treatment temperatures in thefinishing heat treatment step, and presences/absences of the waterwashing-drying steps from Working Example 1 to Working Example 18 andfrom Comparative Example 1 to Comparative Example 3. As the results fromthe XRD measurement, a diffraction pattern corresponding to an α-NaFeO₂type layered structure was obtained from Working Example 1 to WorkingExample 18 and from Comparative Example 1 to Comparative Example 3.

TABLE 1 Heat treatment Heat treatment Composition formula temperature intemperature in of positive electrode second precursor finishing heatWater washing and active material forming step [C. °] treatment step [C.°] drying steps Working Example 1 Li_(1.01)Ni_(0.80)Co_(0.10)Mn_(0.10)O₂575 865 Absent Working Example 2 Li_(1.01)Ni_(0.80)Co_(0.10)Mn_(0.10)O₂600 842 Absent Working Example 3 Li_(1.01)Ni_(0.80)Co_(0.10)Mn_(0.10)O₂600 850 Absent Working Example 4 Li_(1.01)Ni_(0.80)Co_(0.10)Mn_(0.10)O₂600 865 Absent Working Example 5 Li_(1.01)Ni_(0.80)Co_(0.10)Mn_(0.10)O₂600 880 Absent Working Example 6 Li_(1.01)Ni_(0.80)Co_(0.10)Mn_(0.10)O₂600 895 Absent Working Example 7Li_(1.01)Ni_(0.80)Co_(0.125)Mn_(0.075)O₂ 600 850 Absent Working Example8 Li_(1.05)Ni_(0.80)Co_(0.05)Mn_(0.15)O₂ 600 865 Absent Working Example9 Li_(1.00)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 600 755 Absent Working Example10 Li_(1.05)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 600 770 Absent Working Example11 Li_(1.00)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 690 820 Absent Working Example12 Li_(1.00)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 690 850 Absent Working Example13 Li_(1.00)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 690 880 Absent Working Example14 Li_(1.01)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 690 880 Present WorkingExample 15 Li_(1.01)Ni_(0.80)Co_(0.10)Mn_(0.10)O₂ 600 880 PresentWorking Example 16 Li_(1.00)Ni_(0.80)Co_(0.15)Mn_(0.5)O₂ 690 880 PresentWorking Example 17 Li_(1.00)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 690 880Present Working Example 18 Li_(1.00)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 690880 Present Comparative Example 1 Li_(1.01)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂550 755 Absent Comparative Example 2Li_(1.01)Ni_(0.80)Co_(0.15)Mn_(0.05)O₂ 575 755 Absent ComparativeExample 3 Li_(1.00)Ni_(0.80)Co_(0.10)Mn_(0.10)O₂ 600 906 Absent

The following Table 2 shows the amounts of reacted lithium carbonate inthe second precursor forming step in the production step of the positiveelectrode active materials, the amounts of dissolution of the lithiumhydroxide in the positive electrode active materials, and the specificsurface areas from Working Example 1 to Working Example 18 and fromComparative Example 1 to Comparative Example 3. The following Table 3shows the amounts of remaining lithium hydroxide and the amounts ofremaining lithium carbonate on the positive electrode active materialsfrom Working Example 1 to Working Example 18 and from ComparativeExample 1 to Comparative Example 3.

TABLE 2 Amount of reacted lithium carbonate Amount of elution of lithiumhydroxide Specific surface area in second precursor forming step [mass%] in positive electrode active material [mass %] [m²/g] Working Example1 94 0.10 0.38 Working Example 2 97.4 0.14 0.78 Working Example 3 97.40.16 0.54 Working Example 4 97.4 0.09 0.32 Working Example 5 97.4 0.070.24 Working Example 6 97.4 0.06 0.17 Working Example 7 97.5 0.31 0.80Working Example 8 97.2 0.17 0.51 Working Example 9 92.5 0.44 1.19Working Example 10 92.5 0.33 0.94 Working Example 11 99.2 0.13 1.50Working Example 12 99.2 0.08 0.47 Working Example 13 99.2 0.07 0.25Working Example 14 99.2 0.07 0.65 Working Example 15 97.4 0.07 0.64Working Example 16 99.2 0.07 0.53 Working Example 17 99.2 0.07 0.46Working Example 18 99.2 0.07 0.47 Comparative Example 1 91.5 0.45 0.91Comparative Example 2 91.9 0.43 0.99 Comparative Example 3 97.4 0.100.09

TABLE 3 Amount of remaining lithium hydroxide Amount of remaininglithium carbonate on positive electrode active material on positiveelectrode active material Moisture percentage (mass %) (mass %) (ppm)Working Example 1 0.40 0.13 Working Example 2 0.66 0.18 Working Example3 0.48 0.15 Working Example 4 0.36 0.12 Working Example 5 0.28 0.11Working Example 6 0.17 0.10 Working Example 7 1.14 0.35 Working Example8 0.52 0.15 Working Example 9 0.80 0.22 Working Example 10 0.72 0.22Working Example 11 0.59 0.27 Working Example 12 0.46 0.21 WorkingExample 13 0.27 0.18 200 Working Example 14 0.03 0.08 230 WorkingExample 15 0.04 0.09 240 Working Example 16 0.08 0.06 250 WorkingExample 17 0.03 0.06 190 Working Example 18 0.04 0.07 200 ComparativeExample 1 0.83 0.27 Comparative Example 2 0.82 0.25 Comparative Example3 0.25 0.11

As shown in Table 2, in the positive electrode active material ofWorking Example 1, the amount of reacted lithium carbonate in the secondprecursor forming step was about 94 mass %. In the positive electrodeactive materials from Working Example 2 to Working Example 6, theamounts of reacted lithium carbonate in the second precursor formingstep were about 97.4 mass %. In the positive electrode active materialof Working Example 7, the amount of reacted lithium carbonate in thesecond precursor forming step was about 97.5 mass %. In the positiveelectrode active material of Working Example 8, the amount of reactedlithium carbonate in the second precursor forming step was about 97.2mass %. In the positive electrode active materials of Working Example 9and Working Example 10, the amounts of reacted lithium carbonate in thesecond precursor forming step were about 92.5 mass %. In the positiveelectrode active materials from Working Example 11 to Working Example 14and Working Example 16 to Working Example 18, the amounts of reactedlithium carbonate in the second precursor forming step were about 99.2mass %. In the positive electrode active material of Working Example 15,the amount of reacted lithium carbonate in the second precursor formingstep was about 97.4 mass %.

Table 2 shows that the smaller the amount of dissolution of the lithiumhydroxide in the positive electrode active material is, the higher theamount of reacted lithium carbonate in the positive electrode activematerial is and the higher the crystal stability is. Table 3 shows thatthe smaller the amount of remaining lithium carbonate is, the higher theamount of reacted lithium carbonate in the positive electrode activematerial is and the higher the crystal stability is. The amounts ofdissolution of the lithium hydroxide in the positive electrode activematerials from Working Example 1 to Working Example 8 and from WorkingExample 10 to Working Example 18 shown in Table 2 were all low values of0.33 mass % or less, the amounts of reacted lithium carbonate in thepositive electrode active materials were high, and the crystal stabilitywas high.

As shown in Table 2, the amounts of dissolution of the lithium hydroxidein the positive electrode active materials from Working Example 14 toWorking Example 18 on which the water washing had been performedexhibited the low values of 0.07 mass %. Furthermore, as shown in Table3, the amounts of remaining lithium carbonate in the positive electrodeactive materials from Working Example 14 to Working Example 18 on whichthe water washing had been performed exhibited the low values of 0.09 orless. Accordingly, it has been found that performing the water washingensures reducing the amount of remaining lithium carbonate in thepositive electrode active material, reducing the amount of carbonic acidgas generated by lithium carbonate degradation caused by thecharge/discharge cycles, and improving the charge/discharge cyclecharacteristics.

Among the positive electrode active materials from Working Example 14 toWorking Example 18 on which the water washing was performed, thepositive electrode active material of Working Example 17 on which thedrying was performed in two stages was able to reduce the moistureamount to be the least, 190 ppm. Although the drying at the first stagewas not performed on the positive electrode active material of WorkingExample 18, the moisture amount was able to be reduced to 200 ppm.Accordingly, it has been found that, although the drying needs not to beperformed in two stages, the drying at a high temperature of 240° C. for14 hours is effective in that the moisture amount decreases.

In contrast, the positive electrode active materials of ComparativeExample 1 and Comparative Example 2 exhibited the amounts of reactedlithium carbonate in the second precursor forming step of about 91.5mass % and about 91.9 mass %, respectively. Additionally, both ofComparative Example 1 and Comparative Example 2 exhibited the amounts ofdissolution of the lithium hydroxide in the positive electrode activematerials shown in Table 2 higher than 0.33 mass %. Since the heattreatment temperature was low in the second precursor forming step andalso the heat treatment temperature was low in the finishing heattreatment step, the amounts of reacted lithium carbonate lowered inthese Comparative Examples.

The amount of reacted lithium carbonate of Comparative Example 3 was97.4 mass %. Although this Comparative Example had the high heattreatment temperature in the finishing heat treatment step and thereforesatisfied the amount of reacted lithium carbonate, the heat treatmenttemperature in the finishing heat treatment step was high and thedecrease in the specific surface area decreased a reaction area with Li.Consequently, as described later, the resistance increase rates of thelithium ion secondary batteries that used the positive electrode activematerials from Comparative Example 1 to Comparative Example 3 becamehigh and therefore they became unmeasurable. Additionally, the dischargecapacities of the lithium ion secondary batteries using the positiveelectrode active materials from Comparative Example 1 to ComparativeExample 3 were all low.

The following Table 4 shows crystallite diameters of the positiveelectrode active materials, lattice constants of an a-axis, and latticeconstants of a c-axis from Working Example 1 to Working Example 18 andfrom Comparative Example 1 to Comparative Example 3.

TABLE 4 Crystallite Lattice constant Lattice constant diameter of a-axisof c-axis [nm] [Å] [Å] Working Example 1 135 2.87 14.22 Working Example2 138 2.87 14.21 Working Example 3 124 2.87 14.22 Working Example 4 1422.87 14.22 Working Example 5 177 2.87 14.22 Working Example 6 128 2.8714.22 Working Example 7 144 2.87 14.20 Working Example 8 118 2.88 14.23Working Example 9 69 2.87 14.17 Working Example 10 79 2.87 14.20 WorkingExample 11 112 2.87 14.19 Working Example 12 104 2.87 14.21 WorkingExample 13 115 2.87 14.21 Working Example 14 115 2.87 14.21 WorkingExample 15 177 2.87 14.22 Working Example 16 115 2.87 14.21 WorkingExample 17 115 2.87 14.21 Working Example 18 115 2.87 14.21 ComparativeExample 1 79 2.87 14.21 Comparative Example 2 74 2.87 14.22 ComparativeExample 3 — 2.88 14.21

In Table 4, the crystallite diameter indicates the degree of the graingrowth in the positive electrode active material. As long as the ratio(1-x-y-z) of Mn in the composition of the positive electrode activematerial indicated by the above-described Formula (1) is larger than 0,or more preferably 0.05≤(1-x-y-z)≤0.18 is met, the charge and dischargeare possible even when the crystal grains in the positive electrodeactive material grow. The lattice constant indicates that the positiveelectrode active material is correctly produced.

(Evaluation for Compressibility of Positive Electrode Active Material)

Using the autograph, the molding densities at the press pressure of 5MPa were measured as the compressibility of the powder of the positiveelectrode active materials of Working Examples 5, 9, 13, 14, and 15. Theresults were that the positive electrode active material of WorkingExample 5 was 2.3 g/cm³, the positive electrode active material ofWorking Example 9 was 1.7 g/cm³, the positive electrode active materialof Working Example 13 was 2.3 g/cm³, the positive electrode activematerial of Working Example 14 was 2.6 g/cm³, and the positive electrodeactive material of Working Example 15 was 2.6 g/cm³. The following Table5 shows the results of evaluation for compressibility of the positiveelectrode active materials of Working Examples 5, 9, 13, 14, and 15.

TABLE 5 Molding density at press pressure of 5 MPa (g/cm³) WorkingExample 5 2.3 Working Example 9 1.7 Working Example 13 2.3 WorkingExample 14 2.6 Working Example 15 2.6

With the positive electrode active materials of Working Examples 5 and14 on which the heat treatment was performed at the high temperature inthe finishing heat treatment step, the grain growth was promoted,unevennesses on the secondary particle surfaces decreased, and afriction between the secondary particles was lowered; therefore, thecompressibility was satisfactory. Further, in the positive electrodeactive materials of Working Examples 14 and 15 on which the waterwashing and the drying steps were performed, further improvement incompressibility was observed. It is inferred that this occurs becausethe surfaces were modified through the water washing. When the moldingdensity at the press pressure of 5 MPa is 2.5 g/cm³ or more, a densityof a positive electrode mixture layer can be improved, and an effect toincrease the accumulated electric energy per unit volume can besufficiently obtained.

FIG. 3 is a graph illustrating a relationship between the press pressureand the molding density of the positive electrode active materials ofWorking Examples 5, 9, 13, 14, and 15 taking the press pressure [MPa] onthe horizontal axis and the molding density [g/cm³] on the verticalaxis. In the positive electrode active materials of all the embodiments,an approximately rectilinear proportional relationship was observedbetween the press pressure and the molding density at the press pressureof 5 MPa or more; therefore, the compressibility was satisfactory.

(Manufacturing Lithium Ion Secondary Batteries)

Using the positive electrode active materials from Working Example 1 toWorking Example 18 and from Comparative Example 1 to Comparative Example3, the respective lithium ion secondary batteries from Working Example 1to Working Example 18 and from Comparative Example 1 to ComparativeExample 3 were manufactured by the following procedure.

First, the positive electrode active material, a binder, and aconductive material were mixed to prepare positive electrode mixtureslurry. Then, the prepared positive electrode mixture slurry was appliedover an aluminum foil as a positive electrode current collector with athickness of 20 and the positive electrode mixture slurry was dried at120° C. After that, a compression molding was performed with a presssuch that the electrode density became 2.6 g/cm³, and this product waspunched into a disk shape with a diameter of 15 mm to manufacturepositive electrodes. Additionally, negative electrodes were manufacturedusing metallic lithium or lithium titanate (LTO) as negative electrodeactive materials.

Next, lithium ion secondary batteries were manufactured using themanufactured positive electrodes, negative electrodes, and non-aqueouselectrolytes. As the non-aqueous electrolytes, solution produced bydissolving LiPF₆ so as to be the concentration of 1.0 mol/L into asolvent in which ethylene carbonate and dimethyl carbonate were mixedsuch that the volume ratio became 3: 7 was used.

Next, the metallic lithium was used as the negative electrode activematerials for the respective lithium ion secondary batteries fromWorking Example 1 to Working Example 18 and from Comparative Example 1to Comparative Example 3, a charge/discharge test was conducted, and thefirst discharge capacity was measured. With a charging current of 0.2CA, the charge was performed at a constant current and a constantvoltage up to a charge cutoff voltage of 4.3 V. With a discharge currentof 0.2 CA, the discharge was performed at a constant current up to adischarge cutoff voltage of 3.3 V. The following Table 6 shows themeasurement results of the discharge capacities of the lithium ionsecondary batteries from Working Example 1 to Working Example 13 andfrom Comparative Example 1 to Comparative Example 3.

The charge/discharge test was conducted on the respective lithium ionsecondary batteries from Working Example 1 to Working Example 18 andfrom Comparative Example 1 to Comparative Example 3 using lithiumtitanate (LTO) as the negative electrode active materials. With acharging current of 0.2 CA, the charge was performed at a constantcurrent and a constant voltage up to a charge cutoff voltage of 2.75 V.With a discharge current of 0.2 CA, the discharge was performed by thecharge and discharge by two cycles at a constant current up to adischarge cutoff voltage of 1.7 V. Afterwards, an initial DC resistancevalue was measured at a State of Charge (SOC) of 50%. Furthermore, thecharge and discharge were repeated by 100 cycles with the charge anddischarge currents of 3.0 CA, the charge cutoff voltage of 2.85 V, andthe discharge cutoff voltage of 1.7 V. After 100 cycles, a DC resistancevalue with an electric potential at which the initial DC resistancevalue was measured was measured. A percentage of a value found bydividing the DC resistance value measured at the 100th cycle by theinitial DC resistance value was calculated and defined as the resistanceincrease rate.

The following Table 6 shows the measurement results of the resistanceincrease rate of the lithium ion secondary batteries from WorkingExample 1 to Working Example 13 and from Comparative Example 1 toComparative Example 3.

TABLE 6 Discharge capacity Resistance increase rate [Ah/kg] [%] WorkingExample 1 196 30 Working Example 2 197 27 Working Example 3 195 27Working Example 4 195 26 Working Example 5 193 23 Working Example 6 19122 Working Example 7 189 30 Working Example 8 192 27 Working Example 9200 35 Working Example 10 195 33 Working Example 11 198 24 WorkingExample 12 197 29 Working Example 13 192 27 Comparative Example 1 178 47Comparative Example 2 182 41 Comparative Example 3 168 —

As described above, the positive electrode active materials from WorkingExample 1 to Working Example 13 are produced through the above-describedmixing step and firing step, that is, the first precursor forming step,the second precursor forming step, and the finishing heat treatmentstep. The second precursor forming step reacts 92 mass % or more of thelithium carbonate in the first precursor to obtain the second precursor.With the lithium ion secondary batteries from Working Example 1 toWorking Example 13 that used the thus produced positive electrode activematerials from Working Example 1 to Working Example 13 as the positiveelectrodes, the discharge capacity was 189 Ah/kg or more and theresistance increase rate was 35% or less; therefore, all Working Example1 to Working Example 13 obtained the satisfactory results. With thelithium ion secondary batteries from Working Example 14 to WorkingExample 18 that used the positive electrode active materials fromWorking Example 14 to Working Example 18 on which the water washing wasperformed as the positive electrodes as well, the discharge capacity was185 Ah/kg or more and the resistance increase rate was 35% or less;therefore, all Working Example 14 to Working Example 18 obtained thesatisfactory results.

In contrast, the lithium ion secondary batteries of Comparative Example1 and Comparative Example 2 use the positive electrode active materialsof Comparative Example 1 and Comparative Example 2 with the amount ofreacted lithium carbonate in the first precursor of less than 92 mass %as the positive electrodes in the second precursor forming step.Consequently, although the lithium ion secondary batteries ofComparative Example 1 and Comparative Example 2 exhibited thecomparatively high discharge capacity, the resistance increase ratebecame 40% or more. Thus, the properties worsened compared with theresults of the lithium ion secondary batteries from Working Example 1 toWorking Example 13. It has been found that since the discharge capacityof Comparative Example 3 was the capacity lower than those of WorkingExample 1 to Working Example 13, adjusting only the temperature at thefinishing heat treatment step cannot obtain the excellent positiveelectrode active material.

As described above, it was able to be confirmed that Working Example 1to Working Example 18 based on the method for producing the positiveelectrode active material and the positive electrode active material ofthe present invention can obtain the positive electrode active materialthat features the high capacity, the low resistance increase rate, andthe excellent charge/discharge cycle characteristics.

While the embodiments of the present invention have been described indetail with reference to the drawings, the specific configuration is notlimited to the embodiments. Design changes and the like within a scopenot departing from the gist of the present invention are included in thepresent invention.

Reference Signs List [0166]

-   100 Non-aqueous secondary battery (lithium ion secondary battery)-   111 Positive electrode    -   S1 Mixing step    -   S2 Firing step    -   S21 First precursor forming step    -   S22 Second precursor forming step    -   S23 Finishing heat treatment step

1. A method for producing positive electrode active material for lithium ion secondary batteries, the method comprising: a mixing step of weighting and mixing a lithium carbonate and a compound containing respective metallic elements other than Li in Formula (1), Li_(α)Ni_(x)Co_(y)M1_(1−x−y−z)M2_(z)O_(2+B), so as to have a metallic constituent ratio of a composition formula in accordance with the Formula (1) to obtain a mixture, where values in the Formula (1) meet: 0.97≤α≤1.08, −0.1≤β≤0.1, 0.7<x ≤0.9, 0.03≤y≤0.3, 0≤z≤0.1, and 0 <1-x-y-z, M1 is at least one kind of an element selected from the group consisting of Mn and Al, and M2 is at least one kind of an element selected from the group consisting of Mg, Ti, Zr, Mo, and Nb; and a firing step of firing the mixture to obtain a lithium composite compound expressed by the following Formula (1), wherein the firing step includes: a first precursor forming step of performing a heat treatment on the mixture at a heat treatment temperature of 200° C. or more and 400° C. or less for 0.5 hours or more and 5 hours or less to obtain a first precursor; a second precursor forming step of performing a heat treatment on the first precursor under an oxidizing atmosphere at a heat treatment temperature of 450° C. or more and 800° C. or less for 0.5 hours or more and 50 hours or less, the second precursor forming step reacting 92 mass % or more of the lithium carbonate to obtain a second precursor; and a finishing heat treatment step of performing a heat treatment on the second precursor under an oxidizing atmosphere at a heat treatment temperature of 755° C. or more and 900° C. or less for 0.5 hours or more and 50 hours or less to obtain the lithium composite compound.
 2. The method for producing positive electrode active material for lithium ion secondary batteries according to claim 1, wherein the heat treatment temperature in the finishing heat treatment step is 840° C. or more and 890° C. or less.
 3. The method for producing positive electrode active material for lithium ion secondary batteries according to claim 1, wherein the second precursor forming step forms the second precursor such that 97 mass % or more of the lithium carbonate has reacted, the lithium carbonate being contained in the mixture weighted and mixed so as to have the metallic constituent ratio of the composition formula in the Formula (1).
 4. The method for producing positive electrode active material for lithium ion secondary batteries according to claim 1, wherein the heat treatment temperature in the second precursor forming step is 600° C. or more and 700° C. or less.
 5. The method for producing positive electrode active material for lithium ion secondary batteries according to claim 1, wherein the M1 in the Formula (1) is Mn, 0.04≤1-x-y-z≤0.18 being met.
 6. The method for producing positive electrode active material for lithium ion secondary batteries according to claim 1, wherein the lithium composite compound is not water-washed after the finishing heat treatment step.
 7. The method for producing positive electrode active material for lithium ion secondary batteries according to claim 1, the method comprising a step of performing a water washing on the lithium composite compound after the finishing heat treatment step.
 8. The method for producing positive electrode active material for lithium ion secondary batteries according to claim 7, the method comprising a step of performing a drying at least once or more after the step of performing the water washing.
 9. A positive electrode active material for lithium ion secondary batteries expressed by the following Formula (1), Li_(α)Ni_(x)Co_(y)M1_(1−x−y−z)O_(2+β), where values in the Formula (1) meet: 0.97≤a≤1.08, −0.1≤β≤0.1, 0.7<x≤0.9, 0.03≤y≤0.3, 0≤z≤0.1, and 0<1-x-y-z, M1 is at least one kind of an element selected from the group consisting of Mn and Al, and M2 is at least one kind of an element selected from the group consisting of Mg, Ti, Zr, Mo, and Nb; and wherein a specific surface area is 0.10 m²/g or more, and an amount of dissolution (B−A) is 0.33 mass % or less, the amount of dissolution (B−A) being a difference between an amount of lithium hydroxide A and an amount of lithium hydroxide B, the amount of lithium hydroxide A being detected by a neutralization titration after an immersion into a pure water at a solid content percentage of 1.6 mass % for 30 minutes, the amount of lithium hydroxide B being detected by the neutralization titration after an immersion into a pure water at a solid content percentage of 1.6 mass % for 120 minutes.
 10. A positive electrode active material for lithium ion secondary batteries expressed by the following Formula (1), Li_(α)Ni_(x)Co_(y)M1_(1−x−y−z)M2_(z)O_(2+β), where values in the Formula (1) meet: 0.97≤α≤1.08, −0.1≤β≤0.1, 0.7<x≤0.9, 0.03≤y≤0.3, 0≤z≤0.1, and 0<1-x-y-z, M1 is at least one kind of an element selected from the group consisting of Mn and Al, and M2 is at least one kind of an element selected from the group consisting of Mg, Ti, Zr, Mo, and Nb, and wherein a molding density at a press pressure of 5 MPa is 2.5 g/cm³ or more.
 11. The positive electrode active material for lithium ion secondary batteries according to claim 10, wherein the M1 in the Formula (1) is Mn, 0.04≤1-x-y-z≤0.18 being met.
 12. A lithium ion secondary battery, wherein the lithium ion secondary battery uses the positive electrode active material for lithium ion secondary batteries according to claim
 9. 13. A lithium ion secondary battery, wherein the lithium ion secondary battery uses the positive electrode active material for lithium ion secondary batteries according to claim
 10. 14. A lithium ion secondary battery, wherein the lithium ion secondary battery uses the positive electrode active material for lithium ion secondary batteries according to claim
 11. 